1. Field of the Invention
The present invention relates in general to the field of recording media. In particular, this invention provides a recordable/erasable optical storage medium with a single polymer layer and write/read/erase mechanism therefor in which data may be recorded and erased in response to thermal effects and, in particular, in response to light.
2. Description of the Related Art
Optical data storage media in the form of compact disks are well known as an alternative to long-playing records and magnetic tape cassettes. The disks with which consumers are familiar are optical read-only disks, and the common disk player is designed specifically for this type of disk. These disks have a reflective surface containing pits that represent data in binary form. A description of these pits and how they function is provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapter 13.
Compact disks are currently produced by a pressing process similar to the process used to produce conventional long-playing records. The process, referred to herein as the mastering process, starts by first polishing a plain glass optical disk. This disk has an outside diameter from 200 to 240 mm, a thickness of 6 mm and undergoes various cleaning and washing steps. The disk is then coated with a thin chrome film or coupling agent, a step taken to produce adhesion between the glass disk and a layer of photo-resist, which is a photosensitive material. Data on a compact disk master tape are then transferred to the glass disk by a laser beam cutting method.
The glass disk is still completely flat after it is written on by the laser beam because pits are not formed until the glass is photographically developed. The disk surface is first made electrically conductive and then subjected to a nickel evaporation process. The disk, now know as the glass master, then undergoes nickel electrocasting, a process that is similar to that used in making analog phono records. A series of metal replications follow, resulting in a disk called a stamper. The stamper is equivalent to a photographic negative in the sense that it is a reverse of the final compact disk; that is, there are now bumps where there should be pits. This stamper is then used to make a pressing on a transparent polymer such as polyvinyl chloride, poly(ethyl-methacrylate), and polycarbonate. The stamped surface is then plated with a reflective film, such as aluminum or other metal, and finally a plastic coating is applied over the film to form a rigid structure.
The player operates by focusing a laser beam on the reflective metal through the substrate and then detecting reflected light. The optical properties of the substrate, such as its thickness and index of refraction, are thus critical to the player's detection systems and standard players are designed specifically with these parameters in mind.
The pits increase the optical path of the laser beam by an amount equivalent to a half wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. The presence of data takes the form of a drop in intensity of the reflected light. The detection system on a standard player is designed to require greater than 70% reflection when no destructive interference occurs and a modulation amplitude greater than 30% when data is present. These intensity limits, combined with the focusing parameters, set the criteria for the compact disks and other optical data storage media that can be read or played on such players.
Media on which data can be recorded directly on, and read directly from, have a different configuration and operate under a somewhat different principle. One example is described in U.S. Pat. No. 4,719,615 (Freyrer, et al.).
The medium described in Feyrer, et al., includes a lower expansion layer of a rubbery material that expands when heated. The expansion layer is coupled to an upper retention layer that is glassy at ambient temperature and becomes rubbery when heated. Both layers are supported on a rigid substrate. The expansion and retention layers each contain dyes for absorption of light at different wavelengths. Data are recorded by heating the expansion layer by absorption of light from a laser beam at a "record" wavelength to cause the expansion layer to expand away from the substrate and form a protrusion or "bump" extending into the retention layer. While this is occurring, the retention layer rises in temperature above its glass transition temperature so that it can deform to accommodate the bump. The beam is then turned off and the retention layer cools quickly to its glassy state before the bump levels out, thereby fixing the bump.
Reading or playback of the data is then achieved by a low intensity "read" beam that is focused on the partially reflecting interface between the retention layer and air. When the read beam encounters the bump, some of the reflected light is scattered, while other portions of the reflected light destructively interfere with reflected light from non-bump areas. The resulting drop in intensity is registered by the detector. Removal of the bump to erase the data is achieved by a second laser beam at an "erase" wavelength that is absorbed by the retention layer and not by the expansion layer. This beam heats the retention layer alone to a rubbery state where its viscoelastic forces and those of the expansion layer return it to its original flat configuration. The write, read, and erase beams all enter the medium on the retention layer side, passing through retention layer before reaching the expansion layer.
The erasable optical storage medium system described in Feyrer, et al., has a number of disadvantages. For example, the writing and erasure of data must be performed at two different wavelengths of light. Furthermore, the device relies on reflection at the interface between the retention layer and air that results in an inherently low reflectivity (30% maximum) Thus, the system cannot be read by the detection mechanism of a standard compact disk player designed for focusing through a 1.2 mm polycarbonate substrate and requiring 70% reflectance. Additionally, there is either a predetermined level of thermal conductivity between the heated expansion layer, to sufficiently raise the temperature of the retention layer so that it can accommodate the bump formed by the expansion layer, or the retention layer must absorb a predetermined amount of light energy at the "record" wavelength, in order to produce the needed temperature rise in the retention layer during recording. In either case, this requirement must be met and accurately controlled if this media is to be produced with consistent recording characteristics.
In addition, in order for the most effective erasure to be achieved, the retention layer must be heated separately from the expansion layer. This follows from the fact that during erasure the retention layer must reach a rubbery state in order for the viscoelastic forces of a cool expansion layer to pull the expansion layer back to its original flat configuration. If the expansion layer is heated during this time, it will not be in its relaxed state and it will therefore not return to its flat configuration. Because the expansion layer and the retention layers are in intimate physical contact, heat energy must be conducted between the two layers during both the recordation and erase processes, thus negating the possibility of only heating the retention layer. Any attempt to erase the medium during the act of recordation, i.e., direct overwrite data update, therefore would prove unsuccessful.
Copending application Ser. No. 294,723 (assigned to the assignee of the present application) describes an improved optical recording method and apparatus. In one embodiment, the invention includes an expansion layer, a reflective layer, and a retention layer. As the expansion layer is heated it expands, pressing into the thin reflective layer, the retention layer, and a protective layer. In an alternative embodiment, the retention layer is provided between the reflective and expansion layers. The retention layer is pressed into, for example, the protective layer that is sufficiently compliant to allow deformations. The reflective layer is described as being, for example, gallium, aluminum, copper, silver, gold, or indium.
In one embodiment, copending application Ser. No. 357,377 (assigned to the assignee of the present application) describes a liquid reflective layer that is provided adjacent the retention layer opposite the expansion layer. Additionally, improved expansion and retention layers are also described therein.
Optical media described above have one or more limitations. First, two lasers are normally necessary, one for the expansion layer and a second one for the retention layer. These lasers often require individual wavelengths: a "recording laser" emitting a beam with a wavelength corresponding to the absorption frequency of a dye in the expansion layer and an "erasing laser" emitting a beam with a wavelength corresponding to the absorption frequency of a dye in the retention layer. Second, manufacture of these media requires several separate coating operations, thereby increasing the risk of defects due to coating flaws, dust and handling, for example. Also, the manufacturing cost is increased with each additional coating operation.
A purpose of the present invention is to overcome these limitations and to provide recordable/erasable storage media requiring but a single laser for recordation and erasure and also one less coating operation.